U.S. President Donald Trump arrives for a dinner at the Parc du Cinquantenaire during the NATO Summit in Brussels, Belgium July 11, 2018. Geert Vanden Wijngaert/Pool via REUTERS

BRUSSELS (Reuters) - U.S. President Trump significantly toughened his tone on NATO defense spending at a summit on Thursday, singling out Germany, Spain and Belgium in a closed-door session in Brussels, two sources said.

Breaking from a carefully-scripted session to focus on Ukraine and Georgia, Trump “forcibly restated his position on wanting NATO members to reach 2 percent spending target to a short a deadline,” one source said, referring to a NATO goal.